Every type of atom and molecule vibrates at different speeds, giving off light at its own unique set of frequencies. Thus, the chemical composition of an object can be ascertained by studying light that passes through an object. This field of science is known as spectroscopy. The light, when projected on some medium after passing through an object, creates a spectral image. When applied in a two-dimensional world, this technique is known as 2D imaging, or hyper spectral imaging. When spectroscopy and imaging are applied through a three dimensional volume it is known as 3D tomography. This technique is utilized routinely in the medical field in the form of computerized tomography (CT), which uses x-rays to provide a nondestructive or noninvasive method of generating visual sectional views of an object.
In general, a computerized tomography (CT) apparatus performs three processes: scanning, image reconstruction and image display. X-rays are projected over an object positioned between an x-ray source and an x-ray detector. The x-ray source and detector, or object, are moved to scan the entire surface of the object. The detector measures how much of the x-rays penetrate the object to create penetration data that is converted into digital form and stored on a computer. The computer uses the digitized penetration data to create an image of the object by some conversion algorithm and displays the image on a monitor. The displayed image is a function of the density of the object.
In conventional computerized tomography, an x-ray beam in the shape of a fan, and a linear array detector are employed to achieve two-dimensional (2D) imaging. While the data set is complete and image quality is correspondingly high, only a single slice of an object is imaged at a time. When a 3D image is required, a “stack of slices” approach has been employed. Acquiring a 3D data set a 2D slice at a time is inherently tedious and time-consuming.
A more recent approach, based on what is called cone beam geometry, employs a two-dimensional array detector instead of a linear array detector, and a cone beam x-ray source instead of a fan beam x-ray source. At any instant, the entire object is irradiated by the cone beam x-ray source, making cone beam geometry much faster than slice-by-slice scanning using a fan beam or a parallel beam. Also, since each “point” in the object is viewed by the x-rays in 3D rather than in 2D, much higher contrast can be achieved than is possible with conventional 2D x-ray CT. To acquire cone beam projection data, an object is scanned, preferably over a 360-degree angular range, either by moving the x-ray source in an appropriate scanning trajectory, for example, a circular trajectory around the object, while keeping the 2D array detector fixed with reference to the source, or by rotating the object while the source and detector remain stationary. In either case, it is relative movement between the source and object that effects scanning.
Image reconstruction procedures in x-ray CT are often based on the Radon inversion process, in which the image of an object is reconstructed from the totality of the Radon transform of the object. The Radon transform of a 2D object comprises integrals of the object density on lines intersecting the object. The Radon transform of a 3D object comprises planar integrals. The cone beam data, however, are not directly compatible with image reconstruction through inverse Radon transformation, which requires the use of planar integrals of the object as input. Consequently, image reconstruction by inversion from cone beam scanning data generally comprises two steps: (1) convert the cone beam data to planar integrals, and (2) perform an inverse Radon transform on the planar integrals to obtain the image.
Infrared spectroscopy is a technique which is based upon the vibrational changes of the atoms of a molecule. In accordance with infrared spectroscopy, an infrared spectrum is generated by transmitting infrared radiation through a sample of an organic compound and determining what portion of the incident radiation are absorbed by the sample. An infrared spectrum is a plot of absorbance (or transmittance) against wavenumber, wavelength, or frequency. Infrared radiation is radiation having a wavelength between about 750 nm and about 1000 μm. Near-infrared radiation is radiation having a wavelength between about 750 nm and about 2500 nm. Mid-infrared radiation is radiation having a wavelength between about 2500 nm and about 10,000 nm.
A variety of different types of spectrometers are known in the art such as grating spectrometers, FT (Fourier transformation) spectrometers, Hadamard transformation spectrometers, AOTF (Acousto Optical Tunable Filter) spectrometers, diode array spectrometers, filter-type spectrometers, scanning dispersive spectrometers and nondispersive spectrometers.
Filter-type spectrometers, for example, utilize an inert solid heated to provide continuous radiation (e.g. tungsten filament lamp) to illuminate a rotating opaque disk, wherein the disk includes a number of narrow bandpass optical filters. The disk is then rotated so that each of the narrow bandpass filters passes between the light source and the sample. An encoder indicates which optical filter is presently under the light source. The filters filter the light from the light source so that only a narrow selected wavelength range passes through the filter to the sample. Optical detectors are positioned to detect light which either is reflected by the sample (to obtain a reflectance spectra) or is transmitted through the sample (to generate a transmittance spectra). The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis.
An LED Illumination Array can use infrared emitting diodes (IREDs) as sources of near infrared radiation. A plurality (for example, eight) of IREDs are arranged over a sample work surface to be illuminated for quantitative analysis. Near-infrared radiation emitted from each IRED impinges upon an accompanying optical filter. Each optical filter is a narrow bandpass filter which passes NIR radiation at a different wavelength. NIR radiation passing through the sample is detected by a detector (such as a silicon photodetector). The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis.
Acousto Optical Tunable Filter spectrometers utilize an RF signal to generate acoustic waves in a TeO2 crystal. A light source transmits a beam of light through the crystal, and the interaction between the crystal and the RF signal splits the beam of light into three beams: a center beam of unaltered white light and two beams of monochromatic and orthogonally polarized light. A sample is placed in the path of one of the monochromatic beam detectors, which are positioned to detect light which either is reflected by the sample (to obtain a reflectance spectra) or is transmitted through the sample (to generate a transmittance spectra). The wavelength of the light source is incremented across a wavelength band of interest by varying the RF frequency. The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis.
In grating monochromator spectrometers, a light source transmits a beam of light through an entrance slit and onto a diffraction grating (the dispersive element) to disperse the light beam into a plurality of beams of different wavelengths (i.e., a dispersed spectrum). The dispersed light is then reflected back through an exit slit onto a detector. By selectively altering the path of the dispersed spectrum relative to the exit slit, the wavelength of the light directed to the detector can be varied. The amount of detected light is then measured, which provides an indication of the amount of absorbance of the light by the substance under analysis. The width of the entrance and exit slits can be varied to compensate for any variation of the source energy with wavenumber.
Detectors used in spectroscopy generally fall into two classes, photographic detectors, in which radiation impinges upon an unexposed photographic film, and electronic detectors, in which the radiation impinges upon a detector and is converted into an electrical signal. Electronic detectors provide the advantage of increased speed and accuracy, as well as the ability to convert the spectral data into an electronic format, which can be displayed, processed, and/or stored. Examples of electronic detectors include photomultiplier tubes and photodetectors. Photomultiplier tubes are quite sensitive, but are relatively large and expensive. Photodetectors provide the advantage of reduced size and cost. Some examples of photodetectors are pin diode detectors, charge coupled device detectors, and charge injection device detectors.